1. Field of the Invention
This invention relates to molten alkali metal carbonates fuel cells and particularly to porous anodes in contact with alkali metal carbonates electrolytes over long periods of high temperature operation The invention more specifically relates to molten alkali metal carbonates fuel cell porous anodes of nickel, cobalt, and mixtures thereof additionally comprising lithium ferrite and iron. It is important that the total iron content of the anode including the ferrite and the metallic iron of the composite be in excess of about 25 weight percent. The anodes of this invention provide good mechanical properties, good electrical properties and good electrocatalytic properties under molten alkali carbonates fuel cell operating conditions.
2. Description of the Prior Art
Molten carbonates fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, an electrolyte tile making contact with both the electrodes and a cell housing to physically retain the cell components. Under fuel cell operating conditions, in the range of about 500.degree. to about 700.degree. C., the entire electrolyte tile, the carbonate and the inert support material, forms a paste and thus the electrolyte diaphragms of this type are known as paste electrolytes. The electrolyte is in direct contact with the electrodes where the three phase reactions (gas-electrolyte-electrode) take place. Hydrogen is consumed in the anode area producing water, carbon dioxide and electrons. The electrons flow to the cathode through an external circuit producing the desired current flow. At the anode there must be ready entry for the reactant gas, ready exit for the chemical reaction products and ready exit for the product electrons To maintain a high level of stable, long term performance, both electrolyte and electrode design and properties must be optimized and stabilized at the gas-electrolyte-electrode interface.
Porous anodes of cobalt or nickel alloyed with chromium or containing aluminum oxide have been previously used in molten carbonates fuel cells. Improved molten alkali metal carbonates fuel cell porous anodes principally of cobalt or nickel and having a stabilizing agent of chromium, zirconium oxide, or aluminum oxide to maintain porosity and surface area of the anode during fuel cell operation are taught by U.S. Pat. No. 4,247,604. Molten carbonates fuel cell anodes of nickel-chromium are also taught by U.S. Pat. Nos. 4,404,267 and 4,714,586. U.S. Pat. No. 4,317,866 teaches high purity ceria as a molten carbonates fuel cell anode material.
Nickel and iron have been used in specific manners as electrode materials in certain types of fuel cells. U.S. Pat. No. 4,175,153 teaches a hollow fiber structure of nickel and iron having non-porous compact surface layers which are taught to be useful in fuel cell electrodes, particularly in alkali carbonate liquid electrolyte fuel cells. U.S. Pat. No. 3,291,753 teaches fuel cell electrodes which may have a catalytic metal alloy of nickel and iron on a carbon support material. U.S. Pat. No. 4,127,468 teaches a process for preparing porous metal alloy electrodes wherein the metal alloy may broadly include nickel and iron, the electrodes being useful in fuel cells.
U.S. Pat. No. 4,780,437 teaches molten carbonates fuel cell electrodes with elongated pores across the electrode which is fabricated by tape casting forming porous cathodes of porous lithiated nickel oxides or other transition metal oxides possibly combined as oxygenates with lithium, such as LiFeO.sub.2. U.S. Pat. No. 4,564,567 teaches a ceramic fuel cell cathode of doped alkali-metal, transition-metal oxide which may include LiFeO.sub.2, for molten carbonates fuel cells.
A number of patents relate to various electrode materials useful in lithium batteries: U.S. Pat. No. 4,851,306 teaching electrodes formed of lithium alloy/FeS or lithium alloy/FeS.sub.2 and two lithium alloy phases, one of aluminum and the other of aluminum and iron of which up to 50 mol % of the iron may be substituted with nickel or cobalt; U.S. Pat. No. 4,761,487 teaching a Li/Fe/S cathode for batteries; U.S. Pat. No. 4,731,307 teaching lithiated FeS.sub.2 cathode materials for batteries; U.S. Pat. No. 4,340,652 teaching a ternary Li-Fe-O positive electrodes for lithium electrochemical cells; U.S. Pat. No. 4,164,069 teaching Li.sub.2 FeS.sub.2 as a positive electrode metal in an electrochemical cell. U.S. Pat. Nos. 4,158,720 and 4,324,846 teach secondary electrochemical cells having a negative electrode of lithium/aluminum/ferrite. U.S. Pat. No. 4,728,590 teaches secondary electrochemical cells having a positive electrode of iron sulfide, nickel sulfide and lithium sulfide.